Methanol carbonylation process with rhodium catalyst and a lanthanide metal co-catalyst

ABSTRACT

A carbonylation process for making acetic acid using a metallic co-catalyst composition, effective as a rhodium stabilizer and/or rate promoter, at molar ratios of metal/rhodium of about 0.5 to 40. The process includes reacting methanol with carbon monoxide in the presence of a rhodium-based catalytic metal complex with about 1 to 20 weight percent methyl iodide, less than about 8 weight % water and about 0.5 to about 30 weight percent methyl acetate. The crude acetic acid is flashed and further purified. This process is stable in the absence of a lithium iodide cocatalyst, or in low concentrations of lithium iodide, with an STY greater than 10 mol/L/hr.

CLAIM FOR PRIORITY

This application is a national phase entry of International ApplicationNo. PCT/US2010/001698, filed Jun. 14, 2010, entitled “CarbonylationProcess”. The priority of International Application No.PCT/US2010/001698 (PUBLISHED AS WO 2011/159268) is hereby claimed andits disclosure incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates to methanol carbonylation to make aceticacid using an aqueous homogeneous rhodium catalyst medium with one ormore stabilizing and promoting lanthanide-series metals selected andutilized under conditions which generate substantially less than atheoretically equivalent amount of inorganic iodide corresponding to theconcentration of metal added. Metal/rhodium molar ratios of from about0.5:1 to about 30:1 are employed. The process is carried out under lowwater conditions, suitably from about 0.1-10 wt % water in the reactor.Suitable stabilizing and promoting metals are, for example, lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, erbium, thulium, ytterbium, orlutetium.

BACKGROUND

Reaction systems of choice to manufacture acetic acid in high yields, ona large scale with economically viable production rates, include thosewith a relatively low water (less than 14 wt %) aqueous rhodium catalystsystem which includes an iodide salt. See, for example, U.S. Pat. No.5,144,068 to Smith et al. and U.S. Pat. No. 6,657,078 to Scates et al.So called “low water” processes for making acetic acid have much bettercarbon monoxide efficiency than conventional Monsanto processes due, inpart, to less generation of hydrogen and carbon dioxide by way of thewater gas shift reaction.

Commercial systems typically have corrosion metals present in thecatalytic medium which result in relatively low levels of iodide saltsin the presence of methyl iodide under reaction conditions. In general,conventional wisdom is that corrosion metals (i.e., iron, nickel,chromium, molybdenum, and the like) are not as effective as alkalimetals such as lithium in providing inorganic iodide to the system andthereby stabilizing the rhodium catalyst (a significant cost ofproduction) under reduced carbon monoxide pressure as is encountered ina flash vessel. Moreover, corrosion metals have been consideredundesirable due to solubility and by-product issues. See U.S. Pat. No.4,894,477 to Scates et al., Col. 2, line 13 and following, as well asCol. 9, Table 1. As one of skill in the art will be aware, iodide saltcontaining systems are highly effective as to stabilizing the rhodiumfrom precipitating under reduced carbon monoxide partial pressures aswell as maintaining production rates under low water conditions. Therhodium/lithium iodide system has drawbacks, however, notably: (1) thereaction medium is highly corrosive due, in part, to the elevated levelsof iodide salt and (2) the rhodium/lithium iodide system tends togenerate a plethora of aldehyde-related impurities such as propionicacid, acetaldehyde, crotonaldehyde, higher unsaturated aldehydes, andhigher alkyl iodides, all of which are difficult to remove. See Howardet al., Science and Technology in Catalysis 1998, p 64-65 and D. J.Watson, Proceedings of the 17^(th) ORCS Meeting, Marcel Dekker (1998)for more information relating to corrosion and impurities.

Ruthenium and other metals have been considered for their ability topromote higher production rates in combination with iodide salts.Chinese Patent No. 1,562,937 to Haojing Chemical Co., Ltd., disclosesuse of ruthenium as a co-catalyst at a molar ratio to rhodium of 2.9:1,with a water concentration of 3 to 14.5 wt % and a 15.5 wt % iodideconcentration at a rhodium concentration of 1000 ppm (see Table 1). U.S.Pat. No. 5,939,585 to Ditzel et al. disclose use of ruthenium or osmiumas a promoter (Claim 1) at a molar ratio to rhodium range of 0.1:1 to20:1 (Col. 3, lines 58-59) and a water concentration of 0.1 to 7 wt %.U.S. Pat. Nos. 7,368,597 and 7,276,626 both to Gaemers et al.(equivalent to WO 2004/101487 and WO/2004/101488, respectively) show theuse of osmium, rhenium, cadmium, mercury, tungsten, ruthenium or zinc asa rate promoter (§0059) at a molar ratio to rhodium of 0.1:1 to 20:1(§0069) with a water concentration of 0.1 to 30 wt % (§0081). Gaemers etal. also disclose the use of iodide complexes of lanthanide metals,molybdenum, nickel, iron and chromium as stabilizers (§0070). However,Gaemers et al. primarily rely on a ligand to impart catalyst stability.

Other references likewise disclose the use of additional metals in arhodium/iodide catalyst system for making acetic acid. U.S. Pat. No.7,053,241 to Torrence discloses the use of tin or ruthenium in a rangeof molar ratios to rhodium of 0.1:1 to 20:1 (Abstract) at a waterconcentration of 0.1 to 14 weight % (Col. 4, lines 7-15). The processdisclosed in Torrence '241 includes the presence of an iodide ionconcentration greater than about 3 wt % as does most of the literaturediscussing metal promoters/stabilizers in a methanol carbonylationprocess at water concentrations of less than 14% by weight. UnitedStates Publication No. 2008/0071110 to Chen et al., now U.S. Pat. No.7,671,233, for example, show use of lanthanides, copper, titanium,zirconium, vanadium, manganese, cobalt, palladium, tin, chromium,nickel, molybdenum, or zinc (§0014) as a promoter in a range of molarratios to rhodium of about 0.1:1 to about 7:1 (110016 and Examples) at awater concentration of 1 to 14 weight %. Chen et al. also discuss theuse of yttrium in a molar ratio to rhodium range of 0.09:1 to 5:1without another stabilizing component; however, in virtually all cases,significant iodide levels are reported and the apparent intendedfunction of the metal promoter/stabilizer is to stabilize inorganiciodide concentration which, in turn, stabilizes the catalyst solution.

Japanese Kokai Patent Application 2005-336105 to Daicel ChemicalIndustries Ltd., now Japanese Patent No. JP 4657632 B2, discloses amethod for manufacturing carboxylic acid in the presence of a rhodiumcatalyst, lithium iodide at a concentration of 0.1 to 30 wt %, a limitedamount of water (15 wt % or less), and at least one element orelement-containing compound selected from Zn (in a concentration of10-5,000 ppm), Sn, Ge, and Pb (in concentrations of 10-20,000 ppm). U.S.Pat. No. 5,218,143 to Jones shows rhodium catalyzed carbonylation with0.5 to 5 wt % water stabilized with lithium iodide (2-20 wt %;approximately 120:1 to 1200:1 Li:Rh molar ratio) and a Group VI B metalcostabilizer, i.e., chromium, molybdenum, or tungsten, in aconcentration of 0-10,000 ppm which corresponds to a metal:Rh molarratio of approximately 0:1 to 276,000:1. The lithium iodideconcentrations of Jones are significantly higher than those of thepresent invention.

Still other metal iodides have been considered as alternativestabilizers to lithium iodide. For instance, U.S. Pat. No. 5,416,237 toAubigne et al. discloses use of beryllium iodide as a stabilizer, (Col.3, lines 43-49) using up to 10 weight % water.

Various alternatives to rhodium/lithium iodide systems have beensuggested based on laboratory batch unit data, typically includingrelatively low levels of metal salts, generally at equimolar amountswith rhodium or less. In this regard, see Zhang et al., “Promotingeffect of transition metal salts on rhodium catalyzed methanolcarbonylation”, Catalysis Communications 7 (2006), pp. 885-888; Ling etal., “Study of the Effects of Rare Earth Metal Additives on MethanolCarbonylation Reaction”, Hua Xue Tong Bao [Notes of Chemistry], Vol. 68,2005; and Shao et al., “Study of the Effects of Metal Salts on MethanolCarbonylation Reaction”, Journal of Molecular Catalysis (China), Vol.18, No. 6, December, 2004. So also, it has been suggested to useheteropoly acids of molybdenum and tungsten with rhodium catalysts tomake acetic acid, also at relatively low metal concentrations. See Qianet al., “Promoting effect of oxometallic acids, heteropoly acids of Mo,W and their salts on rhodium catalyzed methanol carbonylation”,Catalysis Communications 8 (2007), pp. 483-487. All four documentsprovide experimental data derived from a batch process, with resultsdetermined as soon as 5 minutes into the reaction. These data do notpredict results in a continuous process at equilibrium, nor does thedata supply information concerning the stability of the catalyst systemat reduced carbon monoxide pressure as is seen in a flash vessel of aproduction unit. With respect to Zhang et al., it is noted that,although metal:rhodium molar ratios (Cr, Fe, Ni, and Zn) of from 2.4 to4.7 were considered (Table 1), the rate data were determined after 5minutes. Similarly, with regard to Ling et al., the reaction times wereno higher than 55 minutes (Table 1), and only consider a single promotermolar ratio of 1:1 (Nd, Ce, or La:Rh). Furthermore, Shao et al. againprovided data after only 10 minutes of reaction time (FIG. 1) formetal:rhodium (Sn, Pb, Cr, and Zr) molar ratios of 0.5:1 to 2.5:1. Notethat the tin promoter used was SnCl₂. Finally, Qian et al. provided datacollected after 5 minutes of reaction time (page 484) for HPA:rhodiummolar ratios of from 0.2:1 for phosphotungstic acid (PTA) and sodiumphosphotungstate (SPT) to 6:1 for Na₂MoO₄. In any event, the variouspapers referred to in this paragraph appear to be directed toidentifying metals or metal-containing compositions which provide asubstantial iodide concentration to stabilize the rhodium catalyst.

WIPO Publication WO 2006/064178 to BP Chemicals Limited teaches acatalyst system for the production of acetic acid which comprises arhodium carbonylation catalyst, methyl iodide, and at least onenon-hydrohalogenoic acid promoter, such as a heteropoly acid, in thepresence or absence of alkali metal iodides, alkaline earth iodides orother components, such as amines or phosphine derivatives, recognized ascapable of generating I⁻ by reaction with alkyl iodides such as methyliodides. The WO '178 publication teaches to optionally include acopromoter capable of generating ionic iodide such as lithium iodide,lanthanide metals, nickel, iron, aluminum, and chromium. It is seen inthe Examples which follow that chromium, for example, may be used inaccordance with the present invention without forming inorganic iodideat or near theoretically equivalent amounts corresponding to theconcentration of chromium added, contrary to the teachings of the WO'178 publication.

As the methanol carbonylation process has been practiced at increasinglylower water concentrations other problems have been found to havearisen. Specifically, operating at this new lower water regime hasexacerbated certain impurities in the product acetic acid. As a result,the acetic acid product formed by the above-described low watercarbonylation is frequently deficient with respect to the permanganatetime owing to the presence therein of small proportions of residualimpurities. Since a sufficient permanganate time is an importantcommercial test which the acid product must meet for many uses, thepresence therein of such impurities that decrease permanganate time isobjectionable [Ullman's Encyclopedia of Industrial Chemistry, “AceticAcid”, Volume A1, p. 56, 5^(th) ed]. Of particular concern are certaincarbonyl compounds and unsaturated carbonyl compounds, particularlyacetaldehyde and its derivatives, crotonaldehyde and 2-ethylcrotonaldehyde (also referred to as unsaturated aldehyde impurities).However, other carbonyl compounds known also to affect the permanganatetime are acetone, methyl ethyl ketone, butyraldehyde, and 2-ethylbutyraldehyde. Thus, these carbonyl impurities affect the commercialquality and acceptability of the product acetic acid. If theconcentration of carbonyl impurities reaches only 10-15 ppm, thecommercial value of the product acetic acid will certainly be negativelyaffected. As used herein the phrase “carbonyl” is intended to meancompounds which contain aldehyde or ketone functional groups whichcompounds may or may not possess unsaturation.

It is postulated in an article by Watson, The Cativa™ Process for theProduction of Acetic Acid, Chem. Ind. (Dekker) (1998) 75 Catalysis ofOrganic Reactions, pp. 369-380, that enhanced rhodium catalyzed systemshave increased standing levels of rhodium-acyl species which will formfree acetaldehyde at a higher rate. The higher rate of acetaldehydeformation can lead to the increased production of permanganate reducingcompounds.

The precise chemical pathway within the methanol carbonylation processthat leads to the production of crotonaldehyde, 2-ethyl crotonaldehydeand other permanganate reducing compounds is not well understood. Oneprominent theory for the formation of the crotonaldehyde and 2-ethylcrotonaldehyde impurities in the methanol carbonylation process is thatthey result from aldol and cross-aldol condensation reactions startingwith acetaldehyde. Because theoretically these impurities begin withacetaldehyde, many previously proposed methods of controlling carbonylimpurities have been directed towards removing acetaldehyde andacetaldehyde-derived carbonyl impurities from the reaction system. Soalso, operation at reduced hydrogen partial pressure and/or reducedmethyl iodide has been proposed. See U.S. Pat. No. 6,323,364 to Agrawal,et al., as well as U.S. Pat. No. 6,303,813 to Scates et al., thedisclosures of which are incorporated herein by reference.

Conventional techniques used to remove acetaldehyde and carbonylimpurities have included treatment of acetic acid with oxidizers, ozone,water, methanol, amines, and the like. In addition, each of thesetechniques may or may not be combined with the distillation of theacetic acid. The most typical purification treatment involves a seriesof distillations of the product acetic acid. Likewise, it is known toremove carbonyl impurities from organic streams by treating the organicstreams with an amine compound such as hydroxylamine which reacts withthe carbonyl compounds to form oximes followed by distillation toseparate the purified organic product from the oxime reaction products.However, this method of treating the product acetic acid addssignificant cost to the process.

Despite much effort and substantial need in the art for an improved lowwater, rhodium catalyzed methanol carbonylation process without elevatedlevels of inorganic iodide, little progress has been made and therhodium/lithium iodide system remains the system of choice forcommercial production because of the rhodium stability provided underreduced carbon monoxide pressure as is seen in the flasher of acontinuous production unit.

SUMMARY OF INVENTION

We have unexpectedly found that by judicious choice of alanthanide-series metal co-catalyst composition effective as astabilizer and promoter and utilizing the metal in specified molarratios with rhodium, that a low water acetic acid process can beoperated with inorganic iodide concentrations that are substantiallylower than the theoretically equivalent amount correspondent to theconcentration of the added metals, while preserving catalyst stabilityand achieving high rates.

We have also unexpectedly found that lithium iodide in combination witha lanthanide-series metal, such as samarium, or a lanthanide-seriesmetal combined with another metal, provides effective stabilization andrate promotion at reduced lithium iodide concentrations.

Particularly surprising features of the present invention include thatcertain metals and combinations thereof provide both (1) rhodiumstability under low water conditions especially where reduced carbonmonoxide partial pressure is encountered and (2) elevated acetic acidproduction rates under low water conditions, without maintainingelevated levels of inorganic iodide in the system. The rate promotingand stabilizing compositions may even be added as metal iodides if sodesired; there may be little substantial additional inorganic iodide inthe system due to the addition of such compounds. It is believed thatthe iodide added in such cases is primarily consumed in the system byequilibria, producing methyl iodide and resulting in higher levels ofmetal acetates which provide catalyst stability and promote productionrates.

The benefits of the invention are at least three-fold. First, the lowinorganic iodide levels make the catalyst system less corrosive thanconventional low water catalyst systems, reducing or eliminatingcorrosion problems. Metallurgy requirements for equipment are also lessstringent. Thus, capital and operating costs are reduced. Second, manyof the impurity issues arising from elevated levels of inorganic iodidein the system may be ameliorated or overcome. Without intending to bebound by theory, it is believed that many of the impurities are derivedfrom acetaldehyde, as noted above, which appears to form more readily inthe presence of iodide salts, for example lithium iodide. Acetaldehydeis believed to condense to form unsaturated aldehydes, such ascrotonaldehyde, which then generate higher alkyl iodides in the systemwhich are particularly difficult to remove. So also, acetaldehydeformation appears to cause increases in propionic acid levels because ofthe availability of hydrogen in the reactor. As inorganic iodide levelsare lowered in accordance with the invention, acetaldehyde and relatedimpurities are reduced.

A third benefit arises from the increased production rates at low waterconditions. Without intending to be bound by theory, the metallicco-catalyst composition in the presence of very low inorganic iodideaccelerates the reductive elimination step of acetyl iodide from thecatalyst complex. Consider, for example, a typical depiction of thecatalytic cycle in an acetic acid process as shown in the attached FIG.1, wherein one of skill in the art will appreciate that modification ofkinetics of the various reactions will improve production rates andquality. In particular, accelerating the reductive elimination stepincreases production rates of acetic acid and reduces the opportunityfor aldehyde formation from the rhodium complex, resulting in areduction of acetaldehyde-derived impurities in the final product. Suchimpurities are difficult, if not impossible, to remove withoutextraordinary purification effort.

Still further features and advantages of the invention are apparent fromthe following description.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to theappended drawings. In the Figures:

FIG. 1 is a schematic illustrating interrelated reaction paths for atypical methanol carbonylation process;

FIG. 2 is a schematic diagram of an apparatus suitable for practicingthe process of the present invention;

FIG. 3 is a plot of carbonylation rate versus time for a Rh/Li catalystsystem;

FIG. 4 is a graphical depiction of impurities produced duringcarbonylation (at 5 wt % water except as noted) using lanthanidecompositions as a function of iodide concentration;

FIG. 5 is a graphical depiction of impurities produced duringcarbonylation (at 5 wt % water except as noted) using lanthanidecompositions as a function of production rate;

FIG. 6 is a graphical depiction of impurities produced duringcarbonylation (at 5 wt % water except as noted) using lanthanidecompositions in combination with reduced lithium iodide levels as afunction of iodide concentration; and

FIG. 7 is a graphical depiction of impurities produced duringcarbonylation (at 5 wt % water except as noted) using lanthanidecompositions in combination with reduced lithium iodide levels as afunction of production rate.

DETAILED DESCRIPTION

The invention is described in detail below with reference to numerousembodiments for purposes of exemplification and illustration only.Modifications to particular embodiments within the spirit and scope ofthe present invention, set forth in the appended claims, will be readilyapparent to those of skill in the art.

Unless more specifically defined below, terminology as used herein isgiven its ordinary meaning. Ratios refer to molar ratios, %, ppm andlike terms refer to weight percent, parts per million by weight and soforth, unless otherwise indicated.

When we refer to reaction mixtures or catalyst systems “consistingessentially of” certain components, we mean to exclude other componentsthat would alter the basic and novel characteristics of the composition,that is, substantially change its reactivity, stability or selectivity.The language “consisting essentially of” specifically excludes unlistedsalts in substantial amounts, for example, but does not excludeimpurities, by-products, diluents, and so forth.

For present purposes, a metallic co-catalyst composition is consideredeffective as a rhodium stabilizer in the process at a given waterconcentration if essentially none of the rhodium catalyst metalprecipitates under processing conditions in the flasher of the reactionsection of a carbonylation system for a time sufficient to showcharacteristic stability. To test for stability, a reaction mixture maybe tested under a nitrogen atmosphere in a sealed pressure glass tube at125° C., to simulate the CO partial pressure in the flasher unit.Details of the preparation and procedure appear in U.S. Pat. No.7,053,241, the disclosure of which is incorporated herein by reference.In this test, sealed pressure glass tubes are equipped with controlledtemperature and stirring using a pressure tube reactor system. Thecatalyst solutions are purged with carbon monoxide (CO) at 125° to 150°C. and a pressure of 241.1 kPa with stirring for one hour to ensurecomplete dissolution of the rhodium catalyst complex before conductingcatalyst precipitation tests. The prepared catalyst solutions are cooledand then purged with N₂ for one hour to remove dissolved CO beforeplacing the catalyst solutions into glass tubes which are sealed under aN₂ atmosphere. The rhodium concentration is determined by atomicabsorption (AA) spectroscopy. The rhodium concentration of the solutionis measured 10 minutes after the nitrogen purge is complete, or longerif so desired. Characteristic stability can also be measured by thismethod after 30 minutes, 1 hour, 12 hours, 24 hours, 48 hours, or moreif so desired. If less than 0.5%, and preferably 0, of the rhodiumprecipitates, the system is considered stable, and the metal compositionis deemed effective as a rhodium stabilizer. Alternatively or asadditional indicia of stability, the process may be run under continuousconditions in a carbonylation unit including a flasher, preferably for 5to 6 hours, and the flasher visually inspected for precipitation. In theexperiments discussed herein, turnover in the flasher was 7.5 minutes,which is an appropriate benchmark time for stability under reduced COpartial pressure. Typically, a composition that acts primarily as astabilizer presents a trend of decreased space-time yield in response toincreased molar ratio of the metal in the composition to rhodium in thesystem. An increase in the molar ratio of a co-catalyst which is only astabilizer results in an observed decrease of the space-time yieldbecause of stabilization of rhodium by the co-catalyst, which induces adecrease of the activity of rhodium.

In some embodiments, the determination of stability is performed in asystem having an amount of lithium iodide insufficient to providestability alone. For example, an amount of lithium iodide providing alithium:rhodium molar ratio of 38:1 represents about half of theconventional amount, and cannot provide catalyst stability at thisconcentration, as shown in the examples (i.e., “unstable”).

A metal composition is considered effective as a rate promoter if thecarbonylation rate (space-time yield, or STY defined as the g-moleacetic acid product per volume of reactor solution per time(g-mole/L/hr)) measured in acetic acid production is greater than thatof the same composition without the promoter component or components.The carbonylation rate is also referred to herein as reactionproductivity. Generally, to determine effectiveness of the promoter, thecatalytic system is compared to a like catalyst system with the sameamount of rhodium alone as the catalyst metal. In some embodiments, thecatalytic system is compared to a like catalyst system with the sameamount of lithium iodide co-catalyst and rhodium catalyst metal.Additional acetic acid is added to 100%. Typically, a composition thatacts primarily as a rate promoter, or activator, presents a trend ofincreased space-time yield in response to increased molar ratio of themetal in the composition to rhodium in the system.

As used herein, a transition metal includes Group IIIB to Group IIBmetals; suitable transition metals include titanium, vanadium, chromium,manganese, iron, cobalt, nickel, copper, yttrium, zirconium, molybdenum,ruthenium, lanthanum, hafnium, tungsten, and platinum. The metals ofinterest to this case are the transition metals discussed above as wellas zinc, beryllium, aluminum, strontium, indium, tin, barium, andbismuth as well as HPA compounds discussed further below. The preciseform in which a metal is used is not particularly important, providedthat it is effective as a rate promoter and stabilizer, as discussedabove.

As used herein, HPA refers to heteropoly acids, a class of complexproton acids made up of a metal, oxygen, an element generally from thep-block of the periodic table, and acidic hydrogen atoms. HPAs arestrong Brönsted acids. A heteropoly acid is formed by condensation oftwo or more inorganic oxyacids comprising a coordinated element (polyatom) and a central element (hetero atom). Typically, from two toeighteen poly atoms, oxygen-linked polyvalent metal atoms, surround oneor more hetero atoms. The hetero atom in the heteropoly acid may be oneor more of copper, beryllium, zinc, nickel, phosphorus, silicon, boron,aluminum, germanium, gallium, iron, cerium, cobalt, arsenic, antimony,bismuth, chromium, tin, titanium, zirconium, vanadium, sulfur,tellurium, manganese, platinum, thorium, hafnium, or iodine, and thepolyatom may be one or more of molybdenum, tungsten, vanadium, chromium,niobium, or tantalum, but these examples are not intended to belimiting. These acids include, but are not limited to, phosphomolybdicacid (H₃[PO₄(Mo₂O₆)₆].xH₂O) also known as PMA, tungstosilicic acid(H₄SiW₁₂O₄₀.xH₂O), tungstophosphoric acid (H₃[P(W₃O₁₀)₄].xH₂O) alsoknown as PTA, molybdosilicic acid (H₄SiMo₁₂O₄₀.xH₂O), molybdophosphoricacid (H₃PMo₁₂O₄₀.xH₂O), molybdotungstophosphoric acid(H₃[PMo_(n)W_(12-n)O₄₀].xH₂O), molybdotungstosilicic acid(H₄[SiMo_(n)W_(12-n)O₄₀].xH₂O), vanadotungstophosphoric acid (H_(3+n)[PV_(n)W_(12-n)O₄₀].xH₂O), vanadotungstosilicic acid(H_(4+n)[SiV_(n)W_(12-n)O₄₀].xH₂O), vanadomolybdosilicic acid(H_(4+n)[SiV_(n)Mo_(12-n)O₄₀].xH₂O), vanadomolybdophosphoric acid(H_(3+n)[PV_(n)Mo_(12-n)O₄₀].xH₂O, wherein n is an integer of 1 to 11and x is an integer of 1 or more), tungstoboric acid (H₅BW₁₂O₄₀),molybdoboric acid (H₅BMo₁₂O₄₀) and molybdotungstoboric acid(BH₅Mo₆O₄₀W₆). The structures of some of the well known anions are namedafter the original researchers in this field. The first characterizedand the best known of these is the Keggin heteropolyanion, typicallyrepresented by the formula XM₁₂O₄₀ ^(x-8), where X is the central atom(Si⁴⁺ or P⁵⁺), x is its oxidation state and M is the metal ion (Mo⁶⁺ orW⁶⁺).

Preferably, the HPA selected comprises a phosphorus or silicon heteroatom and at least one polyatom selected from the group consisting oftungsten, molybdenum, chromium, vanadium and tantalum. The preferredHPAs may be represented by the formula H₃M₁₂XO₄₀, where M is thepolyatom, and X is the hetero atom. Especially preferred HPAs comprisepolyatoms selected from tungsten and molybdenum.

In some embodiments, a metallic co-catalyst composition is a compositioncomprising a lanthanide-series metal, metal compound, or metal complex.Lanthanide series metals, or lanthanons, include lanthanum, cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, erbium, thulium, ytterbium, and lutetium. Theprecise form in which a metal is used is not particularly important,provided that it is effective as a rate promoter and stabilizer, asdiscussed above.

A rhodium-based catalyst system refers to a system providing a rhodiummetal catalyst and an iodide promoter in a carbonylation reactionmixture.

An aqueous reaction mixture refers to a carbonylation reaction mixturecomprising, for instance, water, a rhodium-based catalyst, methanoland/or methyl acetate, methyl iodide, a co-catalyst, and acetic acid.

As used herein, “reductive elimination” refers to a catalytic reactionstep of a mechanistic catalyst cycle whereby the rhodium acyl carbonyliodide complex, [Rh(CO)₂I₃(COCH₃)]¹⁻, eliminates acetyl iodide from thecomplex to form acetyl iodide and regenerates the rhodium catalyst,[Rh(CO)₂I₂]¹⁻.

As used herein, “correspondent inorganic iodide” and like terminologyrefers to inorganic iodide attributable to the stabilizing and promotingmetals added to the system in accordance with the invention. Totalinorganic iodide is measured in a reaction medium by titration with anaqueous solution of silver nitrate at room temperature. Titration withsilver nitrate yields a value for total inorganic halides which oftenconsists primarily of inorganic iodides in a system according to theinvention. The measurement is corrected for equilibrium HI levels andinorganic iodides attributable to any corrosion metal iodides which maybe present. That is, inorganic iodide levels due to HI and corrosionmetal iodides are subtracted from total iodides to determine levels ofcorrespondent inorganic iodide attributable to the stabilizing andpromoting metals added to the system in accordance with the invention.In some embodiments, total inorganic iodide is attributable to both thestabilizing and promoting metals and the reduced lithium iodide contentof the reaction mixture. For present purposes “substantially less than atheoretically equivalent inorganic iodide content corresponding to thepresence of a metallic co-catalyst” and like wording refers to areaction mixture in which a metal added as a co-catalyst contributessubstantially less than a theoretically equivalent amount of inorganiciodide determined by assuming that all the metal added would form ametal—iodide ionic compound. For instance, a system in accordance withthe invention in which chromium is used as a stabilizing and promotingmetal would result in a reaction mixture containing substantially lessthan three moles of inorganic iodide for each mole of chromium added(based on a Cr³⁺ oxidation state). Such a reaction mixture wouldgenerally provide an actual inorganic iodide concentration upontitration of less than 75%, 70%, 65%, or 60% of the theoreticallyequivalent inorganic iodide concentration, and typically less than 55%or less than 50% of the theoretically equivalent inorganic iodideconcentration. The reaction mixture would preferably have an actualinorganic iodide concentration of less than 40%, less than 35%, or lessthan 30% of the theoretically equivalent inorganic iodide concentration,and more preferably less than 20% of the theoretically equivalentinorganic iodide concentration. Even more preferably, the reactionmixture would have an actual inorganic iodide concentration of less than15% of the theoretically equivalent inorganic iodide concentration. Aninorganic iodide content of less than 90% or 80% of the equivalentamount corresponding to the metal co-catalyst added may be considered asubstantial reduction in some cases.

Generally, in most cases, a concentration of correspondent inorganiciodide due to the presence of the metallic co-catalyst composition ismaintained in the reactor below 5 wt % or below 4 wt %. In someembodiments, the concentration of correspondent inorganic iodide is dueto the presence of the metallic co-catalyst composition and low levelsof lithium iodide. Typically, the concentration of correspondentinorganic iodide due to the presence of the metallic co-catalystcomposition is maintained in the reactor below 3.5 wt % or below 3 wt %.Preferably, the concentration of correspondent inorganic iodide due tothe presence of the metallic co-catalyst composition is maintained inthe reactor below 2.5 wt % or below 2 wt %. More preferably, theconcentration of correspondent inorganic iodide due to the presence ofthe metallic co-catalyst composition is maintained in the reactor below1.5 wt %. The present invention is advantageously practiced with lessthan 3% or less than 2% or so total inorganic iodides present in thereaction mixture from sources other than the rhodium stabilizing andrate promoting metal composition in any event to ameliorate corrosionproblems and generation of undesirable by-products as noted above.

A rhodium metal catalyst may be added in any suitable form such thatrhodium is in the catalyst solution as an equilibrium mixture including[RhI₂(CO)₂]¹⁻, as is well known in the art. When rhodium solution is inthe carbon monoxide-rich environment of the reactor, solubility of therhodium is generally maintained because rhodium/carbonyl iodide speciesare generally soluble in water and acetic acid. However, whentransferred to carbon monoxide depleted environments as typically existin the flasher, light ends column and so forth, the equilibriumrhodium/catalyst composition changes since less carbon monoxide isavailable; rhodium catalyst precipitates.

An alkyl halide, preferably methyl iodide co-catalyst/promoter (alsoreferred to herein as an iodide promoter) is generally used incombination with the Group VIII metal catalyst component. Methyl iodideis preferred as the alkyl halide promoter. Preferably, the concentrationof alkyl halide in the liquid reaction composition is in the range 1 to50% by weight, preferably 2 to 30% by weight.

While it is preferable in most circumstances to operate with the lowestpossible level of inorganic iodide, in some embodiments it is possibleto operate with additional inorganic iodide which may be added in theform of iodide salts or provided by way of appropriate precursors, forexample lithium acetate, as is known in the art. In most cases, theiodide salts or other inorganic iodide (anion)-generating speciesprovide inorganic iodide in substantially a theoretical amount, as isdescribed herein. The inorganic iodide may be generated in-situ, sinceunder the operating conditions of the reaction system, a wide range ofnon-iodide precursors will react with methyl iodide to generateinorganic iodide, which acts as a catalyst stabilizer. For additionaldetail regarding iodide salt generation, see U.S. Pat. No. 5,001,259 toSmith et al.; U.S. Pat. No. 5,026,908 to Smith et al.; and U.S. Pat. No.5,144,068, also to Smith et al., the disclosures of which are herebyincorporated by reference. Added or generated in-situ inorganic iodidemay be used in connection with this invention as a co-catalyst orco-stabilizer with a metal co-stabilizer, but at a reduced concentrationof inorganic iodide in comparison with conventional carbonylation. Theinorganic iodide-providing co-catalyst may be provided in the form of asoluble salt of an alkali metal or alkaline earth metal or a quaternaryammonium or phosphonium salt. In certain embodiments, the catalyststabilizer/co-promoter is lithium iodide, lithium acetate, or mixturesthereof. The inorganic iodide may be added as a mixture of compounds,such as a mixture of lithium iodide and sodium iodide and/or potassiumiodide. See U.S. Pat. Nos. '259; '908; and '068, all to Smith et al., asreferred to above. Alternatively, the inorganic iodide may be added as aprecursor which generates inorganic iodide in-situ under the operatingconditions of the reaction system. A wide range of non-iodide precursorswhich are useful include alkali metal acetates and carboxylates whichwill react with methyl iodide and/or HI to generate a predeterminedlevel of inorganic iodide. The appropriate level of inorganic iodide mayalso be generated in-situ from non-ionic or neutral precursors, such asa phosphine oxide, arsenes, phosphines, amines, amino acids, sulfides,sulfoxides or any suitable organic ligand or ligands if so desired.Phosphine oxides, phosphines, amines, amino acids or other nitrogen orphosphorous containing compounds and suitable organic ligands generallyundergo reaction readily in the presence of methyl iodide and/or HI atelevated temperatures to yield and maintain a specific level ofinorganic iodide anion concentration in the reaction mixture. Usefulnon-iodide precursors are thus defined by their ability to maintainelevated inorganic iodide anion levels, rather than by the form in whichthey are added to the system. One way of introducing inorganic iodide isby incorporating suitable neutral or ionic precursor moieties such asligands into a rhodium catalyst system as separate entities or complexedwith rhodium (typically monodentate or bidentate ligands). In eithercase, under carbonylation conditions in the presence of methyl iodide,these free ligands, or these ligands complexed with rhodium, decomposeand/or react with methyl iodide and/or HI to provide elevated levels ofinorganic iodide anions. In this regard, the following ChineseReferences are of particular interest: Chinese Publication CN1345631;Application No. 00124639.9; Chinese Publication No. CN1105603;Application No. 94100505.4; and Chinese Publication No. CN1349855;Application No. 00130033.4. Suitable rhodium catalyst complexes whichprovide inorganic iodide as a co-stabilizer/promoter thus includecomplexes having the following structures:

wherein R is H, or a carboxyl-containing hydrocarbon derivative; (X⁻) isBPh₄ ⁻, BF₄ ⁻, or CH₃COO⁻; X is I, Cl, or Br; and n=0, 1, or 2. Othercompounds useful as inorganic iodide-providing co-catalysts includepyridine derivatives such as:

wherein R is H, or a carboxyl-containing hydrocarbon derivative, and nis 0, 1, or 2. Preferably, R is H, or e.g., lithium pyridine-2-formate,lithium pyridine-3-formate, lithium pyridine-4-formate, lithiumpyridine-2-acetate, lithium pyridine-3-acetate, lithiumpyridine-4-acetate, or lithium pyridine-3-propionate. One of skill inthe art will appreciate that a great many other components may be usedto generate inorganic iodide.

The metallic co-catalyst compositions of the invention may be added tothe reaction mixture in any suitable form, preferably wherein thepromoter metal is in a non-zero oxidation state. Following are exemplarychromium salts: Cr(OH)₃.3H₂O, CrCl₂, CrCl₃.6H₂O, CrI₂, CrBr₂, CrI₃.9H₂O,CrBr₃.6H₂O, CrO₃, Cr₂O₃, CrPO₄.6H₂O, Cr(OCOCH₃)₃, Cr(NO₃)₃.9H₂O, CrCO₃,Cr(OCOCH₃)₂. Further details as to suitable chromium compounds are foundin Pauling L., General Chemistry, Chapter 22 pp. 722-740 Dover (1988),the disclosure of which is incorporated herein by reference. Similarforms are suitable for nickel, iron, molybdenum, bismuth, tin, zinc,yttrium, ruthenium, lanthanum, and beryllium. Exemplary lanthanum saltsinclude: La(C₂H₄O₂)₃.H₂O; LaSb; LaAs; LaI₃; La₂(CO₃)₃.8H₂O;La₂(O:C₆Cl₂O₂:O)₃.xH₂O; LaCl₃.7H₂O; LaF₃; La(NO₃)₃.6H₂O;La₂(C₂O₄)₃.9H₂O; La₂O₃; LaP; and La₂(SO₄)₃.9H₂O. Similar forms aresuitable for cerium, praseodymium, neodymium, and the remaininglanthanide series metals. Further details as to suitable lanthanidecompounds are found in the Kirk-Othmer Encyclopedia of ChemicalTechnology, Third Edition, Vol. 19, pp. 833-854, John Wiley & Sons(1982). All precursors capable of forming the active species arepreferred. Acetate, chloride, iodide, carbonyl, and nitrate forms areparticularly preferred.

A preferred carbonylation apparatus or process generally includes atleast a reactive section, including a reactor and a flash vessel, and apurification section. The apparatus of the present invention is used inconnection with the carbonylation of methanol, and/or its reactivederivatives, with carbon monoxide in a homogeneous catalytic reactionsystem comprising a reaction solvent (typically acetic acid), methanoland/or its reactive derivatives, a soluble rhodium catalyst and at leasta finite concentration of water. The carbonylation reaction proceeds asmethanol and carbon monoxide are continuously fed to the reactor. Thecarbon monoxide reactant may be essentially pure or may contain inertimpurities such as carbon dioxide, methane, nitrogen, noble gases, waterand C₁ to C₄ paraffinic hydrocarbons. The presence of hydrogen in thecarbon monoxide and generated in-situ by the water gas shift reaction ispreferably kept low, for example, less than 1 bar partial pressure, asits presence may result in the formation of hydrogenation products. Thepartial pressure of carbon monoxide in the reaction is suitably in therange 1 to 70 bar, preferably 1 to 35 bar, and most preferably 1 to 15bar.

The pressure in the carbonylation reactor is suitably in the range 10 to200 bar, preferably 10 to 100 bar, most preferably 15 to 70 bar. Thetemperature of the carbonylation reaction is suitably in the range 100to 300° C., preferably in the range 125 to 220° C. Acetic acid ismanufactured in a liquid phase reaction at a temperature of from about150-200° C. and a pressure of from about 30 to about 60 bar in typicalembodiments wherein acetic acid is utilized in the reaction mixture asthe solvent for the reaction.

The reaction mixture is fed to a flash vessel at reduced pressure toflash off product and light ends. The pressure in the flash vessel isgenerally less than 2 or 3 bar and usually less than 1 bar. Less than 1bar carbon monoxide partial pressure is also typical in the flashvessel. Less than 0.5 bar or 0.25 bar carbon monoxide partial pressureis used in flash vessels of commercial production units.

Suitable reactive derivatives of methanol include methyl acetate anddimethyl ether. A mixture of methanol and reactive derivatives thereofmay be used as reactants in the process of the present invention.Preferably, methanol and/or methyl acetate are used as reactants. Atleast some of the methanol and/or reactive derivative thereof will beconverted to, and hence present as, methyl acetate in the liquidreaction composition by reaction with acetic acid product or solvent.The concentration in the liquid reaction composition of methyl acetateis suitably in the range 1 to 70% by weight, preferably 1 to 50% byweight, most preferably 2 to 35% by weight.

Water may be formed in-situ in the liquid reaction composition, forexample, by the esterification reaction between methanol reactant andacetic acid product. Water may be introduced to the carbonylationreactor together with or separately from other components of the liquidreaction composition. Water may be separated from other components ofreaction composition withdrawn from the reactor and may be recycled incontrolled amounts to maintain the required concentration of water inthe liquid reaction composition. Preferably, the concentration of waterin the liquid reaction composition is in the range 0.1 to 16% by weight,more preferably 0.1 to 14% by weight, even more preferably 0.1 to 10% byweight, and most preferably 1 to 8% by weight. In some embodiments, theconcentration of water in the liquid reaction composition is generallyin the range of 0.1 to 10 wt %, typically 0.2 to 5 wt %, preferably 0.5to 3 wt %, more preferably 0.5 to 2.5 wt %, even more preferably 0.75 to2.5 wt %, and most preferably 1.5 to 2.5 wt %.

The reaction liquid is typically drawn from the reactor and flashed. Thecrude vapor product stream from the flasher is sent to a purificationsystem which generally includes a light ends column and a dehydrationcolumn, and optionally further purification if required. Thecarbonylation system may use only 2 purification columns and ispreferably operated as described in more detail in U.S. Pat. No.6,657,078 to Scates et al., entitled “Low Energy Carbonylation Process”,the disclosure of which is incorporated herein by reference.

Referring to FIG. 2, there is shown a carbonylation unit 10 of the classutilized in connection with the present invention. Unit 10 includes areactor 12, a flasher 14, a light ends column 16, a drying ordehydration column 18 as well as optionally further purification, suchas a heavy ends column to remove higher boiling impurities (not shown).Reactor 12 includes the reaction medium and there is fed theretomethanol and carbon monoxide. A portion of the reaction medium iscontinuously provided to flasher 14 via line 22 where crude product isflashed and sent to light ends column 16 via line 24 as a hot vaporfeed.

In column 16, the product is purified of light components which exit thecolumn via line 26, are condensed in a first condenser 28 and thendecanted in a decanter 30. Conventionally, the light phase from decanter30 is refluxed to column 16 via line 32, while the heavy phase fromdecanter 30 is returned to the reactor via lines 34, 35. Also provided,but not shown, are absorbers and strippers used to recycle material intothe system.

A purified product stream 40 is withdrawn as a (preferably liquid) sidestream from column 16 and fed to drying column 18 where water is removedfrom the partially purified product. Product is withdrawn via line 42.If necessary, further purification may be done. The overhead and someproduct acetic acid is used as reflux for column 18 or recycled to thereactor via line 44.

Column 16 generates a liquid residue stream 52 which is conventionallyrecycled with flasher residue to the reactor as shown.

EXAMPLES Comparative Experiments

Utilizing a pilot scale apparatus simulating the reactor and flasher ofthe class described above in connection with FIG. 2, a lithium iodidepromoted carbonylation system was compared with a metal promoted systemof the invention. Specifically, Rh(OAc)₃ (1000 ppm Rh) was introducedinto a reactor such as reactor 12 with AcOH/H₂O/HI; the mixture waspressurized under 5 bar of CO at 140° C. during a 1 hour preformationstep. The temperature was then increased in the reactor to 190° C. Atthat time methyl acetate, methyl iodide and the carbonylation catalystprecursor were introduced from a feed tank into the reactor. In a flashvessel such as that shown as flasher 14 of FIG. 2, a mixture ofH₂O/AcOMe/MeI/AcOH was heated to 140° C. When the temperature of thereactor reached 190° C. and the flasher reached 140° C., carbon monoxideand methanol were fed to the reactor along with recycled catalystsolution condensed from the base of the flasher, and the carbonylationreaction began. The flasher volume was replaced every 7.5 minutes at thecirculation rates employed. Acetic acid product, water, methyl acetateand methyl iodide were condensed and collected from the flasheroverhead. The continuous carbonylation process was operated for at leastone hour. The reaction rate was determined by CO uptake measured in thereactor and by the amount of acetic acid collected in the vaporcondensed flasher overhead material. The stability of the catalystsystem was evaluated by measuring the amount of catalyst precipitationin the flasher base after the continuous operation was complete.

Further experimental details and results appear in Table 1 below and inFIG. 3.

TABLE 1 Lithium Iodide Promoted Methanol Carbonylation Rh/LiI systemExperiment 1 Rh reactor concentration (ppm) 989 Lithium/rhodium reactormolar ratio 77.1 Lithium reactor concentration (ppm) 5183 Water reactorconcentration (% wt) 4.2 Acetic Acid concentration (% wt) 75.8 Methyliodide concentration (% wt) 9.7 Methyl Acetate concentration (% wt) 1Methanol feed rate 1.5 g/min Reactor Temperature (° C.) 190 FlasherTemperature (° C.) 142 Time of the experiment (minutes) 500Carbonylation Rate (CO), mol/L/hr 17.3 Carbonylation Rate (AcOH),mol/L/hr 17.8 Iodide reactor concentration (wt %) 9.4 Catalyst precursorused Rh(OAc)₃, 5 wt % Co catalyst precursor used Lil Note: Twocarbonylation rates, one determined by carbon monoxide uptake and onedetermined by quantified acetic acid product, are used to validate theresults by similarity. A third carbonylation rate measuring MeOHconsumed may also be used. The 989 ppm Rh concentration is comparable to1000 ppm used in subsequent experiments. The 77.1 LiI molar ratio iscomparable to the 76 ratio used in many subsequent experiments.

Example Series AA

Following generally the procedures noted above, further experiments wereperformed demonstrating an unstabilized rhodium system at a variety ofwater concentrations, and a lithium iodide-stabilized system at avariety of lithium iodide:rhodium molar ratios and water concentrations.In the examples below, except as otherwise noted, the conditions were asfollows. Methyl iodide was added at a concentration of 10 wt %, andmethyl acetate controlled at a concentration of 2%. Rhodiumconcentrations were 1000 ppm in the reactor. In each case, the balanceof the reaction mixture at the start of the reaction was made up ofacetic acid. Water concentrations ranged from 3 to 8%. With theexception of the rhodium tests without a co-catalyst, the metallicco-catalyst to rhodium molar ratio ranged from 0.5:1 to 115:1. Theratios provided are on a molar basis.

All the tests discussed immediately below were performed at reactionconditions of 190° C. and 30 bar total pressure. The methanolcarbonylation reaction was allowed to proceed from about 2 hours to 9hours.

Runs 2A-2C were operated using rhodium catalyst with no stabilizers(lithium iodide or other) added.

TABLE 2 Rhodium-catalyzed acetic acid production (no stabilizer orpromoter present) H₂O Time run STY Run (wt %) (hr) (mol/l/hr) Stabilityby visual observations 2A 8 3 10.5 Unstable (small amounts of Rhprecipitate into the flasher and the reactor) 2B 6 2 6.6 Unstable(Precipitation of Rh into the flasher and the reactor) 2C 3 3 5.4Unstable (large amounts of Rh precipitate into the flasher and thereactor)

Rh species were preformed from Rh(OAc)₃, at 10 wt % H₂O, 1 wt % HI and89 wt % AcOH at 140° C. and 5 bar CO partial pressure for 1 hour. Nostabilizer was introduced. Table 2 indicates that the methanolcarbonylation reaction rate decreases with a decrease in reactor waterconcentration, which is well known to those skilled in the art. Also, inthe absence of a rhodium catalyst stabilizer, rhodium precipitationincreases as the water decreases.

TABLE 3 Acetic acid production co-catalyzed with lithium iodide. Rh/LiIH₂O Time STY Stability by visual Run Ratio (wt %) run (hr) (mol/l/hr)observations 3A 1/15 5 2.5 11.6 Unstable (Precipitation of Rh into theflasher and the reactor) 3B 1/76 6 3 17.2 Stable (No Rh precipitation)3C 1/76 5 7 17.9 Stable (No Rh precipitation) 3D  1/115 5 9 17.8 Stable(No Rh precipitation)

For experiments presented in Table 3, Rh species were preformed asdescribed for Table 2. LiI was introduced in the flasher medium beforethis material was recycled to the reactor during the beginning of theexperiment prior to methanol carbonylation. When the methanolcarbonylation reaction began, the concentration of water in the reactorwas about 9%. The concentration of water decreased to 5-6% after 1 hourand this water concentration was maintained through the remainder of theexperiment.

Table 3 shows lithium iodide over a wide range of metal/Rh ratios andover a range of water concentrations. In sufficient amounts lithiumiodide stabilizes rhodium effectively. Note that the reaction rate isinfluenced by the lithium iodide. These observations demonstrate theprior art of methanol carbonylation by rhodium promotion and stabilityby iodide salts.

Example Series C Experiments with Rhodium and a Lanthanide Metal (NoInorganic Iodide Providing Co-Catalyst Such as Lithium Iodide was Used)

Following generally the procedures noted above, lanthanum, cerium,praseodymium and so forth were tested to determine their suitability aspromoters and stabilizers. In the examples below, except as otherwisenoted, the conditions were as follows. Methyl iodide was added at aconcentration of 10 wt %, and methyl acetate controlled at aconcentration of 2%. Rhodium concentrations were 1000 ppm in thereactor. In each case, the balance of the reaction mixture at the startof the reaction was made up of acetic acid. All the tests discussedbelow were performed at reaction conditions of 190° C. and 30 bar totalpressure. The ratios provided are on a molar basis.

The methanol carbonylation reaction was allowed to proceed from about 1hour to 10 hours. Carbonylation rates ranged from 13.5 to 18 mol/L/hr.Water concentrations ranged from 5 to 6%. The lanthanide co-catalyst torhodium molar ratio ranged from 5:1 to 40:1.

As demonstrated in the examples below, lanthanide metallicpromoter/rhodium molar ratios of the present invention may besubstantially lower than lithium/rhodium molar ratios by a lithiumiodide promoted system, while exhibiting very similar performance to alithium iodide—stabilized and—promoted system. This feature is believedto make it possible to run the process with less by-product generationthan with conventional promoters.

We have found by titration of the reactor solution with silver nitratethat inorganic iodide levels are extremely low, i.e., 4.1 wt % or less,when using a lanthanide metal as a rate promoter/catalyst stabilizer.

It is seen from the data in the following tables that substituting astabilizing and promoting metal for lithium iodide provides surprisingstability and production rates (STY). Further examples demonstrate likeresults for other compositions of interest.

TABLE C4 Acetic acid production co-catalyzed with lanthanum(LaCl₃•xH₂O). Rh/La H₂O Time run STY Stability by visual Run Ratio (wt%) (hr) (mol/l/hr) observations C4A 1/20 6 8.5 14 Stable (No Rhprecipitation) C4B 1/30 6 9.6 15 Stable (No Rh precipitation) C4C 1/40 53 14.8 Unstable (Solubility concerns with La)

For experiments presented in Table C4, when the preformation of theactive Rh species in the reactor was finished, LaCl₃ was introduceddirectly into the reactor (after cooling and depressurizing thereactor).

Lanthanum provides stability to rhodium in Rh/La molar ratios of lessthan 1/40 and promotes the production rate (STY).

End Run Analyses

To demonstrate that selected co-catalyst metals contribute substantiallyless than equivalent amounts of inorganic iodide to a carbonylationsystem at equilibrium, a series of runs were performed under continuousconditions following generally the procedures noted above. Forcomparison, a run co-catalyzed with lithium iodide was performed asdescribed generally above. In addition, runs were performed usinglanthanum.

In the examples below, except as otherwise noted, the conditions were asfollows. Methyl iodide was added at a concentration of 10 wt %, andmethyl acetate controlled at a concentration of 2%. Rhodiumconcentrations were 1000 ppm in the reactor. The balance of the reactionmixture at the start of the reaction was made up of acetic acid. All thetests discussed below were performed at reaction conditions of 190° C.and 30 bar total pressure. The conditions and results are summarized inTables C5 and C6, below. Table C5 provides data comparable to the dataprovided in the previous examples. Table C6 provides additionalinformation not available for the previous examples.

The runs for Examples C5A-C5B were performed using essentially the sameprocedure as was used for the previous examples. For instance, for runC5B, LaCl₃ was introduced directly into the reactor after thepreformation of the active Rh species in the reactor at the beginning ofthe reaction. At this time, the concentration of water in the reactorwas about 10%. Then the methanol carbonylation reaction began and theconcentration of water decreased to 5-6% after 2 hours. This waterconcentration was maintained through the remainder of the experiment.During this time, the inorganic iodide concentration was less than about4 wt %, which indicates that at the high La concentration, the form ofthe La salt present was not as an iodide salt.

TABLE C5 Acetic acid production co-catalyzed with metallic co-catalysts.Rh/ Time STY Stability Metal H₂O Co- run (mol/ by visual Run Ratio (wt%) catalyst (hr) l/hr) observations C5A 1/76 5 LiI 4 18 Stable C5B 1/305 LaCl₃ 6 14 Stable

Note that all of the co-catalytic lanthanide metal runs presented inTable C5 achieved production rates comparable to the lithium iodide runand provided rhodium stability to the system. A sample of the reactionmixture from each run was titrated for halide (e.g., iodide) content.Corrosion metal content and resultant impurities were also determined.The quantity of corrosion metals is a function of corrosion of thereactor material. The amount varies depending on a variety of factors:run time, temperature, catalysts used and the pre-existing corrosionlevel of the reactor. Thus, it is difficult to compare corrosion metalamounts between runs, or to compare corrosion metal amounts from onesystem to another. The results are shown below.

Theoretically equivalent inorganic iodide content was calculated asfollows. Iodine has a molecular weight of 126.904 g/mol. The theoreticalmass of inorganic iodide was calculated by multiplying the molar ratioof metal to rhodium by the valence of the metal and the molecular weightof iodine. Note that the calculations provided herein use the valence ofthe form of each metallic co-catalyst as it was introduced. An alternatevalence would only be used if it was otherwise clear that the valence ofthe metal changes after introduction to the system. The theoretical massof inorganic iodide was divided by the total mass of the reactionmixture and multiplied by 100 to achieve a theoretical weight percentinorganic iodide content. For example, given a rhodium concentration of1000 ppm, a lanthanum form of LaCl₃ (i.e., valence of 3), and alanthanum:rhodium molar ratio of 30:1, the theoretical inorganic iodidecontent in Run C5B (Lanthanum) was calculated as follows.

Theoretical Mass of Inorganic Iodide:30 mol×valence 3×126.904 g/mol=11421.4 g (based on one mol of Rh)

Total Reaction Mixture Mass (Based on One mol of Rh):

Rh: 102.905 g

Total mass: 1000×mass of Rh: 1000×102.905 g=102905 g

Theoretical Inorganic Iodide Content:(11421.4 g/102905 g)×100=11.0989%

Alternatively, given the mass of lanthanum in weight %, the theoreticaliodide content may be found by multiplying the mass of lanthanum by thevalence and a ratio of the molecular weight of iodine to the molecularweight of chromium. In systems where two metal compositions were added,a theoretical inorganic iodide value was similarly calculated for thesecond metal present in the system, and the theoretical values of iodidefrom the first and second metal compositions were added. Note that thecalculations provided herein use the valence of the form of eachmetallic co-catalyst as it was introduced. An alternate valence wouldonly be used if it was otherwise clear that the valence of the metalchanges after introduction to the system.

The actual iodide content in the system was determined by titration asdescribed above. Note that entries labeled “ND” indicate that theimpurity was not detected. The detection limit for the analytical methodused herein is 10 ppm.

TABLE C6 Iodide concentration, end-run conditions, corrosion metalanalyses and impurities formation for various co-catalysts. TheoreticalIodide Impurities, ppm Co-catalyst Iodide Content titration End Runconditions Corrosion Ethyl- Propi- Concentration (wt %; results H₂OAcOMe MeI Metals, ppm Acetal- Crotonal- crotonal- onic Run (wt %)calculated) (wt %) (wt %) (wt %) (wt %) Fe Ni Mo dehyde dehyde dehydeAcid C5A Li: 0.5 9.1 9 5 1.9 8.5 20 186 163 457 ND ND 166 C5B La: 4 112.1 5 2 10 57 760 332 443 23 ND 26

Without intending to be bound by theory, the propionic acid levels arebelieved to be more representative of the impurity profile achieved byeach composition than is acetaldehyde. Acetaldehyde has a boiling pointof about 20.2° C., in contrast to propionic acid which has a boilingpoint of about 140.7° C. Acetaldehyde and propionic acid flash off withthe product acetic acid (B.P. 118° C.) and are condensed. A fraction ofthe acetaldehyde may be lost as the condensed mixture is removed fromthe condenser as well as during analysis of the impurities.

The results in Table C6 show that metallic co-catalyst compositionsincluding metals such as lanthanum achieve comparable results to arhodium/lithium iodide carbonylation system in terms of STY and catalyststability, but contribute a significantly lower inorganic iodideconcentration to the reaction mixture. Further, the impurity profile,i.e., the relative amounts of each impurity, is profoundly affected bythe metal in each run. See FIG. 4 and FIG. 5 and the data discussedfurther below. When the inorganic iodide of the system is low, suchimpurities are dramatically reduced. Iodide levels below about 5 weight% reduced acetaldehyde levels by about 50%, and propionic acid levels byas much as about 90%, compared to the lithium iodide system results.

To further demonstrate that selected co-catalyst metals contributesubstantially less than equivalent amounts of inorganic iodide to acarbonylation system at equilibrium, an additional series of runs wereperformed under continuous conditions. For comparison, a runco-catalyzed with lithium iodide was performed as described generallyabove. In addition, runs were performed using praseodymium, neodymium,cesium, and several other lanthanide-series metals. In each case, thebalance of the reaction mixture at the start of the reaction was made upof acetic acid. The conditions and results are summarized in Tables C7and C8, below. Table C7 provides data comparable to the data provided inthe previous examples. Table C8 provides additional information similarto Table C6.

The runs for Examples C7A-C7M were performed using essentially the sameprocedure as was used for the previous examples. For instance, for runC7J, LaCl₃ was introduced as discussed for run C5B, above.

Runs that provided unstable systems were not analyzed further.Therefore, no data is provided in Table C8 for run C7M. Also,praseodymium run C7B achieved better results than run C7L, so analysiswas performed only on run C7B.

TABLE C7 Acetic acid production co-catalyzed with metallic co-catalysts.Rh/ Time STY Stability Metal Co- H₂O run (mol/ by visual Run Ratiocatalyst (wt %) (hr) l/hr) observations C7A 1/76 LiI 5 4 18 Stable C7B1/15 Pr(OAc)₃•3H₂O 5 4 17 Stable C7C 1/15 Dy(OAc)3 5 2 16.8 Stable C7D1/15 TbCl3 5 4 16.5 Stable C7E 1/15 Ho(OAc)3 5 1.5 16.5 Stable C7F 1/15Sm(OAc)3 5 3 16.5 Stable C7G 1/15 Yb(OAc)3 5 4 16 Stable C7H 1/15Nd(OAc)₃•3H₂O 5 3 16 Stable C7I 1/15 Gd(OAc)3 5 3 15 Stable C7J 1/30LaCl3 5 4 15 Stable C7K 1/10 CeI₃•2H₂O 5 2.5 13.5 Stable C7L 1/10Pr(OAc)₃•3H₂O 5 3.5 16.5 Stable C7M 1/5  Pr(OAc)₃•3H₂O 5 3.5 15 Unstable

TABLE C8 Iodide concentration, end-run conditions, and impuritiesformation for various co-catalysts. Theoretical Iodide Impurities, ppmCo-catalyst Iodide Content titration End Run conditions Ethyl- Propi-Concentration (wt %; results H₂O AcOMe MeI Acetal- Crotonal- crotonal-onic Run (wt %) calculated) (wt %) (wt %) (wt %) (wt %) dehyde dehydedehyde Acid C7A Li: 0.5 9.1 9 5 1.9 8.5 457 ND ND 166 C7B Pr: 2 5.6 3.54 2 10 250 ND ND 157 C7C Dy: 2.3 5.5 2.7 5 3 10 186 ND ND 27 C7D Tb: 2.35.5 2.5 4 3 11 298 ND ND 178 C7E Ho: 2.3 5.5 2.8 6 4 11 184 ND ND 100C7F Sm: 2.2 5.5 4.1 4 1 10 244 ND ND 13 C7G Yb: 2.5 5.5 2.6 6 7 13 243ND ND 100 C7H Nd: 2.1 5.3 3.1 5 6 15 260 ND ND 30 C7I Gd: 2.3 5.5 3 5 512 211 ND ND 133 C7J La: 4 11 3 6 7 10 353 ND ND 26 C7K Ce: 2 3.6 2.5 64 11 340 ND ND 177

Cerium exhibited relatively low activity. Without being bound to aparticular theory, the relatively high impurity values resulting fromthe cerium co-catalyst are believed to be a result of the low activity.The high propionic acid results reported for terbium and cerium arebelieved to be comparable to the results reported for lithium iodide,with the differences attributable to experimental error.

Make rates for impurities produced in a system according to theinvention may be calculated by multiplying the impurity concentration(ppm) in the acetic acid product by the carbonylation rate, or STY(mol/L/hr), multiplied by the molecular weight of acetic acid (60.05196g/mol), divided by the molecular weight of the impurity, resulting in arate of moles impurity per liter per hour (×10⁻⁶). For propionic acid,the molecular weight is 74.07854 g/mol; for acetaldehyde, the molecularweight is 44.05256. The metallic co-catalysts according to the inventionhave been shown to achieve propionic acid make rates about 2% to about93% lower than a conventional lithium iodide-stabilized system, andacetaldehyde make rates about 28% to about 63% lower than a conventionalsystem.

Other suitable combinations for use in systems having a waterconcentration of 5 wt % or less are listed in Table C9. Run conditionsinclude: 5 wt % water; 2 wt % methyl acetate; 10 wt % methyl iodide; anda concentration of rhodium in the reactor of 1000 ppm. In each case, thebalance of the reaction mixture at the start of the reaction is made upof acetic acid.

TABLE C9 Acetic Acid Production Co-catalyzed with various co-catalysts.Run Rh/Metal 1/Metal 2 Ratio Co-catalyst(s) C9A 1/25/0 Pr C9B 1/25/25La/Co C9C 1/25/5 La/Cu C9D 1/15/0.5 La/Zn C9E 1/25/0.5 La/Sn C9F1/15/0.5 Nd/Zn C9G 1/25/15 Nd/Sn C9H 1/25/3 Nd/Ru C9I 1/25/1 Nd/In C9J1/15/3 Pr/Ru C9K 1/25/0.5 Pr/Sn C9L 1/25/0.5 Pr/Zn C9M 1/15/0.5 Pr/HPAC9N 1/15/0.5 Sm/Sn C9O 1/25/5 Sm/HPA C9P 1/25/3 Sm/Ru C9Q 1/15/25 Y/La

As a result primarily of the experimentation described above, lanthanum,cerium, praseodymium, neodymium, samarium, gadolinium, terbium,dysprosium, holmium, and ytterbium are considered to be both activatorsand stabilizers.

Screening experiments performed in a batch system have shown promise forseveral additional cocatalysts provided in an Rh:Metal molar ratio of1/15, as shown in Table C10. For comparison, a reference MeOAcconversion value was determined for a rhodium/lithium iodide system of55%.

TABLE C10 Metal cocatalyst screening results MeOAc % Cocatalystconversion Er 45 Eu 45 Lu 40 Tm 45Experiments with Rhodium, Lithium Iodide, and a Lanthanide Metal

Following generally the procedures noted above, runs were performedusing lithium iodide in combination with a variety of lanthanide metals.In the examples below, except as otherwise noted, the conditions were asfollows. Methyl iodide was added at a concentration of 10 wt %, andmethyl acetate controlled at a concentration of 2%. Rhodiumconcentrations were 1000 ppm in the reactor. In each case, the balanceof the reaction mixture at the start of the reaction was made up ofacetic acid. All the tests discussed below were performed at reactionconditions of 190° C. and 30 bar total pressure. The ratios provided areon a molar basis.

The methanol carbonylation reaction was allowed to proceed from about 2hours to 4 hours. Carbonylation rates ranged from 16 to 18 mol/L/hr.Water concentrations ranged from 2 to 5%. The lithium iodide to rhodiummolar ratio ranged from 15:1 to 76:1. The lanthanide co-catalyst torhodium molar ratio ranged from 15:1 to 25:1.

The conditions and results are summarized in Tables C11 and C12, below.Table C11 provides data comparable to the data provided in the previousexamples. Table C12 provides additional information similar to TableC10.

The runs for Examples C11A-C11E were performed using essentially thesame procedure as was used for the previous examples. For instance, forExample C11B, Sm(OAc)₃ was introduced through a feed tank into thereactor after the preformation of the active Rh species in the reactorat the beginning of the reaction. At this time, the concentration ofwater in the reactor was about 18%. LiI was introduced in the flashermedium before this material was recycled to the reactor during thebeginning of the experiment and prior to methanol carbonylation. Thenthe methanol carbonylation reaction began and the concentration of waterdecreased to about 5% after 2 hours and this water concentration wasmaintained through the remainder of the experiment.

Runs C11D and C11E demonstrate the applicability of the invention tosystems operated at very low water levels.

TABLE C11 Acetic acid production co-catalyzed with metallicco-catalysts. Rh/LiI/ Time STY Stability Metal Co- H₂O run (mol/ byvisual Run Ratio catalyst (wt %) (hr) l/hr) observations C11A 1/76 LiI 54 18 Stable C11B 1/15/15 LiI; Sm(OAc)₃ 5 3 17.2 Stable C11C 1/15/15 LiI;Pr(OAc)₃ 5 2 16 Stable C11D 1/76 LiI 2 4 16.5 Stable C11E 1/15/15 LiI;Pr(OAc)₃ 2 2 16.6 Stable

TABLE C12 Iodide concentration, end-run conditions, and impuritiesformation for various co-catalysts. Theoretical Iodide Impurities, ppmCocatalyst Iodide Content titration End Run conditions Ethyl- Propi-Concentration (wt %; results H₂O AcOMe MeI Acetal- Crotonal- crotonal-onic Run (wt %) calculated) (wt %) (wt %) (wt %) (wt %) dehyde dehydedehyde Acid C11A Li: 0.5 9.1 9 5 1.9 8.5 457 ND ND 166 C11B Li: 0.1; Sm:2.2 7.3 2.9 4 3 13 299 ND ND 25 C11C Li: 0.1; Pr: 2 7.3 3.5 5 2 10 272ND ND 29 C11D Li: 0.5 9.1 8 2.1 2.5 9.1 297 ND ND 246 C11E Li: 0.1; Pr:2 7.3 2.6 3.0 4.1 11.1 196 ND ND 88

Without being bound to a particular theory, some lanthanide metalsappear to provide better results in combination with lithium iodide,whereas other lanthanide metals appear to work better alone. Samariumacts more strongly as a promoter than as a stabilizer. The additionalstability provided by the lithium iodide of run C11B resulted in animproved production rate with a relatively small increase in impurityproduction. In contrast, praseodymium is a strong stabilizer. Theadditional stability provided by lithium iodide in run C11C decreasedactivity and resulted in a lower production rate. Run C11E was haltedafter 2 hours due to equipment blockage by crystallized praseodymium.Run C11E demonstrates that the method according to the invention can besuccessfully performed at low water concentrations. Without being boundby theory, it is believed that the level of acetaldehyde decreases withdecreasing water concentration because there is less hydrogen availablein the reaction medium. Note FIG. 6 and FIG. 7.

Combinations of lithium iodide with lanthanum, neodymium, and higherconcentrations of praseodymium are also believed suitable for use withwater concentrations of 5 wt % or below. Suitable amounts for lithiumiodide include about 15 times the amount of rhodium present on a molarbasis in conjunction with suitable lanthanon metal amounts such as about25 times the amount of rhodium present on a molar basis.

There is provided in one aspect of the invention, a continuous processfor the production of acetic acid utilizing lanthanide series metals.The process comprises reacting a compound, selected from the groupconsisting of methanol and reactive derivatives thereof, with carbonmonoxide to produce acetic acid in an aqueous reaction mixture, thereaction being carried out while maintaining a concentration of water inthe reaction mixture of from 0.1 wt % up to about 8 wt %, the reactionalso being carried out in the presence of a homogeneous rhodium-basedcatalyst system comprising: (i) a rhodium catalyst metal; (ii) an iodidepromoter; and (iii) a metallic co-catalyst composition including a metalselected from the group consisting of lanthanide series metals, andmixtures thereof, and (b) recovering acetic acid from the reactionmixture, wherein the process is controlled and the metallic co-catalystcomposition is selected so that it is effective as a stabilizer and arate promoter, and the reaction mixture contains substantially less thana theoretically equivalent inorganic iodide content corresponding to thepresence of the metallic co-catalyst composition. Generally, the amountof inorganic iodide present is less than 75% of the theoreticallyequivalent inorganic iodide content corresponding to the presence of themetallic co-catalyst composition. Typically, the amount of inorganiciodide present is less than 65% of the theoretically equivalentinorganic iodide content corresponding to the presence of the metallicco-catalyst composition but still less is preferable in many cases, forexample, wherein the amount of inorganic iodide present is less than55%, 40%, or 30% of the theoretically equivalent inorganic iodidecontent corresponding to the presence of the metallic co-catalystcomposition. On a weight basis, the process is operated with aninorganic iodide content in the reaction mixture of less than 4.5 weight% or with less than 3.5 weight % or less than 3 weight % or still morepreferably with an inorganic iodide content in the reaction mixture ofless than 2.5 weight %. These features are independently combined, forexample, as noted below. In some cases the amount of water in thereaction mixture is generally maintained from about 0.1 weight percentup to less than 8 weight percent, and in some cases up to 10% by weightof the reaction mixture.

The process including a lanthanide co-catalyst may be operated whereinthe amount of inorganic iodide present is less than 75% of thetheoretically equivalent inorganic iodide content corresponding to thepresence of the metallic co-catalyst composition and the inorganiciodide content in the reaction mixture is less than 4.5 weight % orwherein the amount of inorganic iodide present is less than 65% of thetheoretically equivalent inorganic iodide content corresponding to thepresence of the metallic co-catalyst composition and the inorganiciodide content in the reaction mixture is less than 3.5 weight % such aswherein the amount of inorganic iodide present is less than 65% of thetheoretically equivalent inorganic iodide content corresponding to thepresence of the metallic co-catalyst composition and the inorganiciodide content in the reaction mixture is less than 3 weight %. In apreferred aspect, the amount of inorganic iodide present is less than65% of the theoretically equivalent inorganic iodide contentcorresponding to the presence of the metallic co-catalyst compositionand the inorganic iodide content in the reaction mixture is less than2.5 weight %.

The process including a lanthanide co-catalyst may be furthercharacterized by a reaction productivity (STY) greater than 10moles/L/hr and/or wherein the iodide promoter is methyl iodide and/orwherein the water content of the reaction mixture is maintained at lessthan 7 weight %. In some cases, the water content of the reactionmixture is maintained at a level of from 1 weight % to 7 weight %, whilein still other cases the water content of the reaction mixture ismaintained at a level of from 2 weight % to 6 weight % or from 0.2weight % to 5 weight % or

from 3 weight % to 5 weight %. The water content of the reaction mixturemay be maintained at less than 3 weight % or from 0.5 weight % to 3weight %. Other operating water levels for the reaction mixture include:wherein the water content of the reaction mixture is maintained at alevel of from 1 weight % to 2.75 weight %; or wherein the water contentof the reaction mixture is maintained at a level of from 1.5 weight % to2.5 weight %. The process may be operated with a total inorganic iodidecontent in the reaction mixture of less than 4.5 weight % and the watercontent of the reaction mixture is maintained at a level of less than 5weight % or the process may be operated with a total inorganic iodidecontent in the reaction mixture of less than 4 weight % and the watercontent of the reaction mixture is maintained at a level of less than 5weight %. Still other preferred modes of operation include: wherein theprocess is operated with a total inorganic iodide content in thereaction mixture of less than 3.5 weight % and the water content of thereaction mixture is maintained at a level of less than 5 weight %; orwherein the process is operated with a total inorganic iodide content inthe reaction mixture of less than 3 weight % and the water content ofthe reaction mixture is maintained at a level of less than 5 weight %;or wherein the process is operated with a total inorganic iodide contentin the reaction mixture of less than 2.5 weight % and the water contentof the reaction mixture is maintained at a level of less than 5 weight%; or wherein the process is operated with a total inorganic iodidecontent in the reaction mixture of less than 2.5 weight % and the watercontent of the reaction mixture is maintained at a level of from 0.5 toless than 5 weight %.

The process including a lanthanide co-catalyst may be carried outwherein the metallic co-catalyst composition is provided withoutproviding additional inorganic iodide, especially wherein the metallicco-catalyst composition comprises a metal selected from the groupconsisting of cerium, praseodymium, europium, gadolinium, terbium,erbium, thulium, ytterbium, and lutetium or wherein the metallicco-catalyst composition comprises lanthanum, neodymium, samarium,dysprosium. In most cases, the metal is present in a metal:rhodium molarratio of at least 5:1 and up to 40:1 such as wherein the metal ispresent in a metal:rhodium molar ratio of at least 10:1 and up to 35:1or wherein the metal is present in a metal:rhodium molar ratio of morethan 10:1 and up to 30:1. In a preferred embodiment, the metal ispresent in a metal:rhodium molar ratio of at least 10:1 and up to 20:1.

The process including a lanthanide co-catalyst may be carried outwherein the reaction mixture further comprises an inorganiciodide-providing second co-catalyst in an amount that alone isinsufficient to stabilize the rhodium catalyst. In a preferredimplementation, the inorganic iodide-providing second co-catalyst islithium iodide. In most cases, the inorganic iodide-providing secondco-catalyst is present in an amount less than 76 times the amount ofrhodium present on a molar basis. The inorganic iodide-providing secondco-catalyst may be present in an amount less than 50 times the amount ofrhodium present on a molar basis or less such as wherein the inorganiciodide-providing second co-catalyst is present in an amount less than 40times the amount of rhodium present on a molar basis or wherein theinorganic iodide-providing second co-catalyst is present in an amountless than 25 times the amount of rhodium present on a molar basis.Generally, the inorganic iodide-providing second co-catalyst is presentin an amount of from 1 to 50 times the amount of rhodium present on amolar basis. In still other cases the inorganic iodide-providing secondco-catalyst is present in an inorganic iodide-providing:rhodium molarratio of from 0.5:1 to 40:1 which includes those embodiments wherein theinorganic iodide-providing second co-catalyst is present in an inorganiciodide-providing:rhodium molar ratio of from 10:1 to 20:1. Preferredlanthanides may include samarium or praseodymium. The metal isadvantageously present in a metal:rhodium molar ratio of at least 5:1and up to 35:1 such as wherein the metal is present in a metal:rhodiummolar ratio of at least about 10:1 and up to about 30:1.

The molar ratio of metal/rhodium in the reaction mixture is suitably atleast 0.5/1 in many implementations of the inventive process.

The process including a lanthanide co-catalyst may be carried outwherein wherein the reaction mixture contains from 250 ppm rhodium to3000 ppm rhodium such as from 500 ppm rhodium to 2000 ppm rhodium. Themetallic co-catalyst composition is added to the reaction mixture in the+1, +2, +3, +4, +5 or +6 oxidation state.

Yet another particularly useful commercial embodiment is a continuousprocess for the production of acetic acid comprising: (a) reacting acompound selected from the group consisting of methanol and reactivederivatives thereof, with carbon monoxide to produce acetic acid in anaqueous reaction mixture disposed in a pressurized reactor at elevatedpressure, the reaction being carried out in the presence of ahomogeneous rhodium-based aqueous catalyst system comprising: (i) arhodium catalyst metal; (ii) methyl iodide maintained in said reactionmixture in a concentration of from about 1 to about 20 weight percent;and (iii) a metallic co-catalyst composition selected from the groupconsisting of the lanthanide series metals, and mixtures thereof; (iv)water maintained in said reaction mixture in a concentration of from 0.1weight percent up to less than 8 weight percent; (v) methyl acetatemaintained in said reaction mixture in a concentration of from about 0.5to about 30 weight percent; and (vi) acetic acid; (b) providing a streamof the reaction mixture to a flash vessel at a reduced pressure; (c)flashing crude acetic acid product from the reaction mixture to generatea crude product stream including acetic acid, methyl acetate, methyliodide and water; and (d) purifying the crude product stream to removemethyl acetate, methyl iodide and water therefrom, to obtain a purifiedacetic acid product, wherein the process is controlled and the metallicco-catalyst composition is selected so that the metallic co-catalystcomposition is effective as a stabilizer and a rate promoter, and thereaction mixture contains substantially less than a theoreticallyequivalent inorganic iodide content corresponding to the presence of themetallic co-catalyst composition. This process also has the features andcomponents in the amounts recited above and is preferably furthercharacterized by a propionic acid concentration of less than 150 partsper million as measured at the flash vessel overhead. More preferably,the process is further characterized by a propionic acid concentrationof less than 100 parts per million as measured at the flash vesseloverhead such as characterized by a propionic acid concentration of lessthan 50 parts per million as measured at the flash vessel overhead orcharacterized by a propionic acid concentration of less than 30 partsper million as measured at the flash vessel overhead. A partial pressureof carbon monoxide is usually maintained above 1 bar in the pressurizedreactor, while a partial pressure of carbon monoxide is usuallymaintained below 1 bar in the flash vessel.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. In view of the foregoing discussion, relevantknowledge in the art and references discussed above in connection withthe Background and Detailed Description, the disclosures of which areall incorporated herein by reference, further description is deemedunnecessary. In addition, it should be understood that aspects of theinvention and portions of various embodiments may be combined orinterchanged either in whole or in part. Furthermore, those of ordinaryskill in the art will appreciate that the foregoing description is byway of example only, and is not intended to limit the invention.

The invention claimed is:
 1. A continuous process for the production ofacetic acid comprising: (a) reacting a compound, selected from the groupconsisting of methanol and reactive derivatives thereof, with carbonmonoxide to produce acetic acid in an aqueous reaction mixture, thereaction being carried out while maintaining a concentration of water inthe reaction mixture of from 0.1 wt % up to about 8 wt %, the reactionalso being carried out in the presence of a homogeneous rhodium-basedcatalyst system comprising: (i) a rhodium catalyst metal; (ii) an iodidepromoter; and (iii) a metallic co-catalyst composition effective as astabilizer and a rate promoter, said composition including a metalselected from the group consisting of lanthanide series metals andmixtures thereof, and (b) recovering acetic acid from the reactionmixture; wherein the metal of the metallic co-catalyst composition ispresent in a molar ratio to rhodium of from at least 5:1 up to 50:1 andwherein further the reaction mixture contains less than 75 percent of atheoretical amount of inorganic iodide equivalent to the presence of themetallic co-catalyst composition.
 2. The process according to claim 1,wherein the process is operated with an inorganic iodide content in thereaction mixture of less than 4.5 weight %.
 3. The process according toclaim 1, wherein the process is further characterized by a reactionproductivity (STY) greater than 10 moles/L/hr.
 4. The process accordingto claim 1, wherein the iodide promoter is methyl iodide.
 5. The processaccording to claim 1, wherein the metallic co-catalyst composition isprovided without providing additional inorganic iodide.
 6. The processaccording to claim 5, wherein the metal is present in a metal:rhodiummolar ratio of at least 5:1 and up to 40:1.
 7. The process according toclaim 1, wherein the reaction mixture further comprises an inorganiciodide-providing second co-catalyst in an amount that alone isinsufficient to stabilize the rhodium catalyst.
 8. The process accordingto claim 7, wherein the inorganic iodide-providing second co-catalyst islithium iodide.
 9. The process according to claim 7, wherein theinorganic iodide-providing second co-catalyst is present in an inorganiciodide-providing:rhodium molar ratio of from 0.5:1 to 40:1.
 10. Theprocess according to claim 1, wherein the reaction mixture contains from250 ppm rhodium to 3000 ppm rhodium.
 11. The process according to claim1, wherein the aqueous reaction mixture is disposed in a pressurizedreactor at elevated pressure, wherein the homogeneous rhodium-basedcatalyst system further comprises methyl acetate maintained in saidreaction mixture in a concentration of from about 0.5 to about 30 weightpercent; and acetic acid; and wherein further the step of recoveringacetic acid comprises providing a stream of the reaction mixture to aflash vessel at a reduced pressure; flashing crude acetic acid productfrom the reaction mixture to generate a crude product stream includingacetic acid, methyl acetate, methyl iodide and water; and purifying thecrude product stream to remove methyl acetate, methyl iodide and watertherefrom, to obtain a purified acetic acid product.
 12. The processaccording to claim 11, wherein the process is further characterized by apropionic acid concentration of less than 150 parts per million asmeasured at the flash vessel overhead.
 13. The process according toclaim 11, wherein a partial pressure of carbon monoxide is maintainedabove 1 bar in the pressurized reactor.
 14. The process according toclaim 11, wherein a partial pressure of carbon monoxide is maintainedbelow 1 bar in the flash vessel.
 15. The process according to claim 1,wherein the amount of inorganic iodide present is less than 40 percentof the theoretical amount of inorganic iodide corresponding to the metalof the co-catalyst composition.
 16. The process according to claim 1,wherein the lanthanide series metal is selected from the groupconsisting of lanthanum, cerium and ytterbium.
 17. The process accordingto claim 1, wherein the lanthanide series metal is lanthanum.
 18. Theprocess according to claim 1, wherein the lanthanide series metal isselected from the group consisting of praseodymium, neodymium,promethium, samarium, europium, gadolinium, terbium, dysprosium,holmium, erbium, thulium and lutetium.
 19. The process according toclaim 1, wherein the lanthanide series metal is samarium.
 20. Theprocess according to claim 1, wherein the lanthanide series metal isdysprosium.
 21. The process according to claim 1, wherein the lanthanideseries metal is neodymium.
 22. The process according to claim 1, whereinthe lanthanide series metal is praseodymium.
 23. The process accordingto claim 1, wherein the lanthanide series metal is holmium.
 24. Theprocess according to claim 1, wherein the metal is present in ametal:rhodium molar ratio of at least about 10:1 to less than 40:1.